There is a lot of it, and lots available in trade paperback.
The proper reading order is:
The Time Monster, 2000 AD prog 330 (Aug. 1983). Story by Pat Mills, art by Angie Mills.
The Beast in the Broch, 4 episodes, 2000 AD progs 331-334 (Aug. to Sept. 1983). Story by Pat Mills, art by Massimo Belardinelli.
Warrior's Dawn, 2000 AD prog 335 (Sept. 1983). Story by Pat Mills, art by Mike McMahon.
The Beltain Giant, prog 336 (Oct. 1983). Story by Pat Mills, art by Mike McMahon.
Bride of Crom, 6 episodes, progs 337-342 (Oct. to Nov. 1983). Story by Pat Mills, art by Massimo Belardinelli.
Creeping Death, prog 343 (Nov. 1983). Story by Pat Mills, art by Massimo Belardinelli.
The Bull Dance, prog 344 (Nov. 1983). Story by Mills, art by Belardinelli.
Hero's Blood, 3 episodes, progs 345-347 (Dec. 1983). Story by Mills, art by McMahon.
The Shoggey Beast, 4 episodes, progs 348-351 (Dec. 1983 to Jan. 1984). Story by Mills, art by McMahon.
Sky Chariots, 9 episodes, progs 352-360 (Jan. to Mar. 1984). Story by Mills, art by McMahon.
Dragonheist, 7 episodes, progs 361-367 (Mar. to May 1984). Story by Mills, art by Belardinelli.
The Battle of Clontarf, 1985 Annual (Aug. 1984). Story by Mills, art by Belardinelli.
Time Killer, 22 episodes, progs 411-428 and 431-434 (Mar. to Sept. 1985). Story by Mills, art by Glenn Fabry (pts 1-2, 9-11 & 17-19), David Pugh (pts 3-8, 12-16 & 20-22) and Bryan Talbot (inks, pt 19).
Tomb of Terror, 15 episodes, progs 447-461 (Dec. 1985 to Mar. 1986). Story by Mills, art by Glenn Fabry (pts 1-2 & 12-14) and David Pugh (pts 3-11 & 15).
The Devil's Banquet, 1986 Sci-Fi Special (June 1986). Story by Mills, art by Glenn Fabry.
The Spoils of Annwn, 7 episodes, progs 493-499 (Oct. to Dec. 1986). Story by Mills, art by Mike Collins & Mark Farmer.
Slaine the King, 12 episodes, progs 500-508 and 517-519 (Dec. 1986 to Apr. 1987). Story by Mills, art by Glenn Fabry.
The Killing Field, prog 582 (July 1988). Story by Angela Mills, art by Fabry.
"the mini-series," 3 episodes, progs 589-591 (Aug. to Sept. 1988). Story by Pat Mills, art by Fabry.
The Arrow of God, 1989 Annual (Sep. 1988). Story by Mills, art by Steve Parkhouse.
The Horned God, Book I, 10 episodes, progs 626-635 (May to July 1989). Story by Mills, art by Simon Bisley.
The Horned God, Book II, 10 episodes, progs 650-656 and 662-664 (Oct. 1989 to Jan. 1990). Story by Mills, art by Bisley.
The Horned God, Book III, 11 episodes, progs 688-698 (July to Sept. 1990). Story by Mills, art by Bisley.
Slaine the High King, 1992 Yearbook (Sep. 1991). Story by Mills, art by Glenn Fabry.
The Jealousy of Niamh, 2 episodes, progs 850-851 (Sept. 1993). Story by Mills, art by Greg Staples & Nick Percival.
Demon Killer, 8 episodes, progs 852-859 (Sept. to Oct. 1993). Story by Mills, art by Glenn Fabry (pts 1-7) and Dermot Power (pt 8).
The Return of the High King, Slaine Poster Prog #1. Story by Mills, art by Dermot Power.
The Queen of Witches, 8 episodes, progs 889-896 (May to July 1994). Story by Mills, art by Power.
The Name of the Sword, 7 episodes, progs 950-956 (July to Sept. 1995). Story by Mills, art by Greg Staples.
Lord of Misrule, 10 episodes, progs 958-963 and 995-998 (Sept. 1995 to June 1996). Story by Mills, art by Clint Langley.
The Bowels of Hell, prog 1000 (July 1996). Story by Mills, art by Jim Murray.
The Treasures of Britain, Book I, 10 episodes, progs 1001-1010 (July to Sept. 1996). Story by Mills, art by Dermot Power.
The Cloak of Fear, 2 episodes, progs 1011-1012 (Oct. 1996). Story by Mills, art by Steve Tappin.
The Treasures of Britain, Book II, 8 episodes, progs 1023-1031 (Jan. to Feb. 1997). Story by Mills, art by Power.
The Demon Hitch-hiker, prog 1032 (Mar. 1997). Story by Mills, art by Tappin.
King of Hearts, 7 episodes, progs 1033-1039 (Mar. to Apr. 1997). Story by Mills, art by Nick Percival.
The Grail War, 10 episodes, progs 1040-1049 (Apr. to July 1997). Story by Mills, art by Tappin.
Secret of the Grail, 10 episodes, progs 1090-1099 (Apr. to June 1998). Story by Mills, art by Tappin.
Lord of the Beasts, prog 1100 (24 pgs.) (June 1998). Story by Mills, art by Rafael Garres.
Kai, 4 episodes, progs 1104-1107 (July to Aug. 1998). Story by Mills & Gallagher, art by Paul Staples.
The Banishing, 2 episodes, progs 1108-1109 (Aug. 1998). Story by Mills & Gallagher, art by Wayne Reynolds.
The Triple Death, prog 1111 (10 pgs.) (Sep. 1998). Story by Mills, art by Reynolds.
The Swan Children, 3 episodes, progs 1112-1114 (Sep. to Oct. 1998). Story by Mills, art by "Siku" (Ajibayo Akinsiku).
Macha, 4 episodes, progs 1115-1118 (Oct. to Nov. 1998). Story by Mills, art by Paul Staples.
Beyond, "Prog 2000" (Dec. 1999). Story by Mills, art by Greg Staples.
The Secret Commonwealth, 17 episodes, progs 1183-1999 (Mar. to July 2000). Story by Mills, art by David Bircham.
Moloch, 6 episodes, "Prog 2003" & 1322-1326 (Dec. 2002 to Feb. 2003). Story by Mills, art by Clint Langley. (Episodes 1 and 6 double-length.)
Golamh, 6 episodes, progs 1350-1355 (July to Aug. 2003). Story by Mills, art by Clint Langley. (Episodes 1 and 6 double-length.)
Scota, 7 episodes, "Prog 2004" & 1371-1376 (Dec. 2003 to Feb. 2004). Story by Mills, art by Clint Langley. (Episode 1 double-length.)
Tara, 7 episodes, 2000 AD "Prog 2005" & 1420-1425 (Dec. 2004 to Feb. 2005). Story by Mills, art by Clint Langley. (Episode 1 double-length.)
Odacon, 7 episodes, 2000 AD progs 1436-1442 (Apr. to June 2005). Story by Mills, art by Clint Langley. (Episode 7 double-length.)
Carnival, 8 episodes, 2000 AD "Prog 2006" and progs 1469-1475 (Dec. 2005 to Feb. 2006). Story by Mills, art by Clint Langley.
There are 26 graphic novels compiling a lot (but not all) of therese - look at http://www.2000ad.org/?zone=covers&page=character&choice=SLAINE&Comic=graphicnovels
Hope that helps